The present invention is generally directed to baluns and, more particularly, to baluns which are implemented as part of a multilayer structure.
A balun (which term comes from the phrase BALanced to UNbalanced) is a passive three port electronic circuit used for conversion between symmetrical and nonsymmetrical transmission lines. The signal, for example incoming to a nonsymmetrical port, is divided between two symmetrical ports providing signals which have the same amplitude but with phases which are 180 degrees offset relative to one another on their outputs. Baluns are used, for example, in the construction of balanced amplifiers, mixers and antenna systems.
The balun construction depends on the intended operating frequency range. In the microwave frequency range, where the size of the structure is comparable to the wavelength of the signal, distributed element circuit technology is commonly used. In lower frequency ranges, e.g., up to 2500 MHz, coupled wire transformer solutions are common in which wires are wound spirally around a highly permeable magnetic core. These conventional balun configurations suffer from a number of problems.
These transformer solutions, using the phenomenon of magnetic coupling between wires, are theoretically wide band circuits. In practice, however, compensation for eigen capacitances is needed, especially in the frequency range 400 to 2500 MHz. This means that the physical construction of the transformer-type baluns has to be specifically optimized for operation within its operating frequency bandwidth. Additionally, it is difficult to accurately set the length of the wires to be wound about the core so that baluns which are designed to be the same, actually have substantially the same electrical characteristics.
Most existing baluns operating in the high (e.g., greater than 2500 MHz) frequency range, give good balun performance only if both symmetrical ports are well matched. In many applications, power matching of the symmetrical ports is not desirable for other reasons. For example, power matching on the symmetrical inputs of mixers or amplifiers worsens their noise parameters. Thus, a compromise between power matching and noise matching is needed.
Moreover, ongoing miniaturization of electronic structures is, in turn, causing the miniaturization of baluns. For example, baluns used in the frequency range 400 to 2000 MHz are not usually bigger than about 20 mm.sup.2 and are designed for automatic surface mounting onto the end product. However, during the production of the baluns themselves, manual mounting is still used because the wires require manual winding around the core and the ends of the wires need to be inserted into electrical connectors on the end product. Manual mounting is expensive, time consuming and causes spread in the parameters of the end product, e.g., a radio receiver, of which the balun is just one of many components.
Thus, it would be desirable to provide a balun having better symmetry when working with unmatched loading, which do not require different physical constructions to handle different operating frequency ranges, and which are less expensive to manufacture by allowing automatic integration of the balun with other circuit components as opposed to manual mounting.